DNA Transcription Prokaryotic Initiation Similarities between DNA Replication and DNA Transcription Before we begin our discussion on prokaryotic transcription, it is helpful to first point out some similarities and differences between the process of DNA replication and DNA transcription. The processes that synthesize DNA and RNA are similar in that they use similar nucleotide building blocks. They also use the same chemical method of attack by a terminal -OH group of the growing chain on the triphosphate group of an incoming nucleotide. Both replication and transcription are fueled by the hydrolysis of the pyrophosphate group that is released upon attack. There are, however, a number of important differences between these two distinct processes. The Structure of RNA Polymerase There are two main segments of the RNA polymerase molecule: the core enzyme, and the sigma subunit. These two pieces are together referred to as the "holoenzyme". The core enzyme is itself composed of a beta, beta prime, and two alpha subunits; together the core is responsible for
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